Many people with enduring mental health difficulties (EMHD) have reduced social connections and subsequent experiences of isolation and loneliness. This paper outlines the development and trial of a manualised social-identity-based group therapy intervention to increase social connection and develop social skills to build connections outside of healthcare settings. This intervention was developed in line with the quality intervention development (6SQuID) methodology. The paper describes initial data from a feasibility study (which was ceased prematurely due to COVID-19 restrictions). Semi-structured outcome interviews were conducted with the participants and analysed using thematic analysis. Uptake, retention and acceptability of participants was high. Four primary themes were identified across the dataset: Therapeutic Process, Social Identity, Generic Components of Group Therapy and Impact of COVID-19. Participants experienced positive changes in their social behaviour, mood and avoidance. Some were unsure of the concept of social identity while reporting positive changes. A theoretically coherent manualised treatment was developed and tested on a small scale. This study supports social identity-based CBT groups as a potentially effective way of increasing social connection in those with EMHD. Social identity theory supported participants to positively integrate an area of potential social threat into their self-concept and may offer a positive addition to current psychological interventions